A downhole survey instrument typically consists of electronics and various sensors packaged within a chassis usually machined from a cylindrical piece of non-magnetic material. The downhole survey instrument is usually a long, slender, assembly, due to the form factor of drilling sensor apparatus in which it is used. These typical instruments also have features that retain and ruggedize the sensors and electronics due to the harsh downhole environment. The survey assemblies typically include an inclinometer which sometimes consists of three or more, typically orthogonally positioned, accelerometers. An accelerometer pair is usually mounted normal to the elongated chassis axis, and a third accelerometer, usually called the Z axis accelerometer, is typically coaxial to the instrument chassis long axis.
In most typical systems, the ‘Z’ axis accelerometer for a downhole survey sensor is installed at one end of the elongated chassis since the cross-section of the accelerometer consumes a large percentage of the available chassis cross section. Furthermore, the chassis material cross-section must provide the structural integrity needed to maintain the relative positions of the sensors in the assembly through handling and use in the harsh downhole environment.
The typical configuration consists of installing an accelerometer into a coaxial pocket machined into one end of the instrument chassis and then retaining it with a threaded member or members that clamp the accelerometer in place. The disadvantage of this scheme is that it 1) limits placement of the ‘Z’ axis accelerometer near one end of the assembly; 2) If the ‘Z’ axis accelerometer is not located at the end of the chassis, additional members may need to be attached which complicates the sensor; 3) it makes routing of conductors difficult as not much cross-section remains for thru-wireways in the chassis; and 4) it takes more time to replace accelerometer due to the complexity of the assembly so that, for example, end components would need to be removed/un-wired to access the accelerometer.
Another configuration simply uses an elastomeric compound to ‘glue’ the accelerometer in place. This has the disadvantage of poor stability (since the accelerometer may be able to move) and difficult replacement/rework since the “glue” must be removed.
Thus, it is desirable to provide a mount for downhole survey sensor that overcomes the limitations and drawbacks of typical systems and it is to this end that the disclosure is directed.